Calcium sulfate scale, commonly called gypsum, is sometimes present in carbonate/dolomite formations that undergo primary, secondary and tertiary oil production. Secondary dissolution of calcium sulfate in the formation, by fresher waters, can carry the calcium sulfate-rich water to the wellbore and near-wellbore formation, where gypsum scale can precipitate. These scales cause severe problems when they occur in downhole equipment, tubing, and perforations in the casing adjacent the formation. Calcium carbonate scales are easily removed by pumping acid solutions, usually 10-15% hydrochloric acid (HCl), downhole to dissolve the scale. However, acid solutions are not effective in removing gypsum scales from the wellbore and/or formation. The commonly used method to remove gypsum is two-part: pumping an inorganic or organic chemical downhole to convert the scale to an acid-soluble compound; and to dissolve the converted scale with dilute acid solutions. Such a converter technique is disclosed in U.S. Pat. Nos. 2,787,326 and 4,155,857. U.S. Pat. No. 3,651,868 also discloses a two stage method of removing calcium sulfate deposits comprising first contacting said deposits with a pressurized solution of ammonium and carbon dioxide in water and then contacting it with an aqueous acid solution. These methods are time consuming and not cost effective for removing scale, and addition the acid is corrosive to metal equipment and hazardous to handle.
The present invention provides an improved method for removing calcium sulfate deposits from wells which combines a one-step chemical conversion of the calcium sulfate scale to calcium carbonate and subsequent in situ dissolution of the calcium carbonate scale without using mineral acids such as hydrochloric acid or nitric acid for dissolution.